Research into catalytic reactions has a long history stretching as far back as ammonia synthesis (a nitrogen fixation method) at the beginning of the 20th century. In the present day, catalysts are employed in a wide range of applications, such as synthesis of feedstocks for chemical products such as daily living products and the like, and of pharmaceuticals, food products, pesticides and fertilizers, and the like. Catalysts also play a role in reducing carbon dioxide emissions from factories and exhaust gases from automobiles, and there are high expectations that catalysts may provide possible solutions to serious environmental problems of global scale, such as the problems of global warming and acid rain.
A catalytic reaction proceeds on the surface of a solid, and it is therefore necessary to expand the surface area in order to increase the reaction efficiency. In cases in which costly noble metals such as platinum are used as the active catalyst, the catalyst is typically used in the form of nanoparticles measuring 1 to 100 nm in size, supported on the surface of a porous carrier such as alumina, silica gel, ceramic, or the like.
In recent years, due to the development of microscopy techniques having atomic resolution, new nanostructures have been discovered, and nano-scale electronic phenomena, such as quantum effects, are finally becoming understood. There has also been dramatic progress in nanotechnology research aimed at active commercialization and exploitation of the unique material characteristics found in these nano regions. Nanotechnology research, dubbed “nanotech,” has attained a measure of general name recognition, but examples of successful instances of commercialization of nanotech on a commercial scale are extremely limited. This is due to the complex production techniques needed to design nanostructures, and to the extreme difficulty in handling them due to their microscopic nanosize. For example, for carbon nanotubes (CNT), which show promise in the future as component materials for electronic devices or mechatronic systems, and which have actually already been commercialized in some areas and are the subject of ongoing competitive research and development efforts, synthesis techniques on the meter level have finally been developed for multilayer CNT (Non-patent document 1), and on millimeter-unit lengths for single layer CNT (Non-patent document 2), although these are in an arrayed form.
As regards the mechanical characteristics of CNT, an astounding value of 63 GPa for tensile strength, measured by an atomic force microscope, has been reported. The strength measurement was made on CNT of short length; about 10 μm (Non-patent document 3). Similar results have been ascertained from transmission electron microscope observation as well, and since no constriction is observed to accompany the rupture process, it was reported that defect sites contribute to rupture (Non-patent document 4). With monocrystal silicon nanowires, which have the highest potential application as transistor elements, length extension remains difficult, and this limitation, which is a problem common to crystalline nanowires, remains a real impediment to commercialization of nanowires.
A critical point in terms of overcoming the length problem stems from the fact that all one-dimensional nanowires to date are composed of crystal phases. Crystalline materials, even those of nano size, typically contain various defect sites such as dislocations, point defects, twinning, grain boundaries, and the like, and the presence of these defect sites exerts considerable influence when extending the length of high-strength nanowires.
In contrast, the metallic glass nanowires discovered by the inventors (Non-patent document 5, Patent Document 1) are composed of amorphous structures, and therefore can retain high strength free from the influence of defect sites such as dislocations. A further advantage is that superplastic working in a supercooled liquid zone unique to vitreous materials can be utilized, making it possible to fabricate high-strength nanowires of lengths measured in millimeter units or greater, which have been difficult to do with past nanowires based on crystalline materials. Furthermore, such metallic glass has excellent functionalities due to its alloy structure, and the design of nanostructures having such excellent functionalities represents a significant contribution to the technical development not only of catalysts, but also of high-performance devices, precision engineered machinery, and the like.
The Inoue group at Tohoku University successfully stabilized a supercooled liquid state of metallic glass exhibiting a distinct glass transition of the sort not observed in normal metals or amorphous alloys, to create a “bulk metallic glass” of very large size, which has attracted attention worldwide as a novel basic material developed in Japan. Because metallic glass does not have dislocations, excellent material characteristics, such as extremely strong resistance to plastic deformation, ultra-high strength, high elastic extension, low Young's modulus, high corrosion resistance, and the like are attained. According to the most recent report, Zr-based bulk metallic glass 30 millimeters in diameter has been successfully produced (Non-patent document 6).
Meanwhile, the mechanical characteristics of these metallic glass materials sufficiently meet requirements as to mechanical strength in precision micro-machinery and micromachine components, and commercialization of these materials has been proceeding at a rapid pace in recent years. A material for the world's smallest geared motor incorporating a high-precision gear (0.3 mm in diameter) has been commercialized (Non-patent document 7). In fatigue tests of this motor, a lifespan 100 times longer than that of a steel gear (SK4) was reported.
As a method for manufacturing these metallic glass nanowires, the inventors previously filed a patent application (Patent Document 2) for a metallic glass nanowire manufacturing method wherein metallic glass of ribbon or rod form, secured at top and bottom at the ends thereof and with the bottom end capable of being placed under traction, is placed with the bottom end thereof under traction within an oxidation-preventive atmosphere; a step of either (a) bringing a mobile heated filament into contact on the vertical with the metallic glass specimen of ribbon or rod form, (b) with electrodes secured at the top and bottom ends, passing current therethrough, interrupting the current just prior to rupture, or (c) heating the metallic glass specimen of ribbon or rod form with a laser, is carried out, to force-heat the metallic glass into a supercooled liquid state; and the metallic glass nanowire formed thereby is force-cooled, to thereby maintain the metallic glass state of the nanowire. However, this method is unsuited to mass production.
Another known method for manufacturing acicular particles efficiently from an amorphous alloy is a method involving employment of a cylindrical body having a recess created through partial recession of the inside wall surface, in which a molten metal of a composition capable of being quench hardened into an amorphous alloy is dripped or sprayed towards a coolant flow, the coolant flow being generated by circulation of a coolant along the inside wall surface of the cylindrical body. However, the metal powder manufactured by this method has an average outside diameter of 10 μm and average length of about 1 mm, whereas a smaller diameter and greater length would be more desirable for employment as a catalyst (Patent Document 3).
In cases in which metallic glass nanowires are employed as catalysts, it is desirable to employ a plurality of reduced-diameter metallic glass nanowires, in order to increase the specific surface area. However, while it is necessary to employ a metallic glass nanofiber composed of a plurality of metallic glass nanowires in order to increase the catalytic activity, to the extent that the diameters have been reduced to the nano level, the metallic glass nanowires are difficult to handle, resulting in the problem that it is difficult to manufacture the individual metallic glass nanowires, and to then form a metallic glass fiber for utilization as a catalyst.